Optical refrigeration can include laser excitation of rare-earth doped glass and crystal host material systems. In some systems, optical refrigeration can include use of ytterbium doped fluorozirconate glass (Yb:ZBLAN).
In modern interpretation, the cooling cycle is based on conversion of low-entropy low-energy input optical field (laser) into an isotropic higher-energy spontaneous emission (fluorescence). Excitation laser is red-shifted from the mean spontaneous emission energy of the transition (λf). Following absorption, out of equilibrium excitation gets thermalized within the ground and excited state manifolds of the rare-earth ion. This is accomplished by phonon absorption from the lattice vibrations of the host. Thus thermal quanta of energy kT are carried away from the host in a form of spontaneously emitted photons, leading to cooling of the material.
Heat generating, non-radiative recombination pathways are not desirable, demanding high quantum efficiency (ηeqe) materials. Rare-earth ions (e.g. Ytterbium) exhibit metastable transitions in the lowest energy levels, satisfying high quantum efficiency requirement. Additional requirement is of host purity. Impurities introduce extrinsic heat generating recombination pathways, e.g. transition-metal impurity, which is manifested in reduction of absorption efficiency, ηabs, defined below. Both of these requirements are captured in expression for cooling efficiency, defined as a ratio of cooling power to absorbed power:
                                          η            c                    ⁡                      (                          λ              ,              T                        )                          =                                            P              cool                                      P              abs                                =                                                                      η                  eqe                                ⁡                                  [                                      1                                          1                      +                                                                        α                          b                                                /                                                                              α                            r                                                    ⁡                                                      (                                                          λ                              ,                              T                                                        )                                                                                                                                ]                                            ⁢                              λ                                                      λ                    f                                    ⁡                                      (                    T                    )                                                                        -            1                                              (        1        )            where λ denotes laser wavelength, αb is the background absorption coefficient while αr (λ) resonant absorption coefficient of Yb ions. The product in brackets is denoted as absorption efficiency ηabs. In practical considerations, material sheds a kBT of thermal energy per excitation, which means that the product ηeqeηabs>1−kBT/hvF, where νF=c/λF, c—speed of light. For room temperature Yb emission this product has to be larger than 96.8%, while at 100K, larger than 99%. Since ηeqe is governed by intrinsic recombination mechanisms for a given material, one must improve ηabs in order to obtain low temperature operation of an optical refrigerator.
The standard approach to increase absorption efficiency has been to lower background absorption by purifying material growth process and starting materials. While this is an important task, increase of resonant absorption has been largely ignored, due to the fundamental reason. As material cools resonant absorption in the anti-Stokes tail falls exponentially, according to Boltzmann law. It is mainly this reason that halts cooling at low temperatures.
Vibration-free cryogenic operation in ytterbium-doped fluoride crystals has been reported. The record is cooling to 93K from room temperature. While numerous applications benefit from this technology, one application stands out which concerns ultrastable lasers using reference external cavities made of monolithic single crystal silicon cooled to 124K. The vibration-free nature of optical refrigeration and the temperature range of operation render this application desirable.